netfilter: nf_conntrack: allow to register bridge support

This patch adds infrastructure to register and to unregister bridge
support for the conntrack module via nf_ct_bridge_register() and
nf_ct_bridge_unregister().

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>